With the development of the long-distance transmission technology and the access technology, at present, a dumbbell type is presented in network development, and a metropolitan-area capacity becomes a bottleneck for further improving a network capacity. A large-capacity metropolitan-area transmission solution needs to be cost-effective while meeting a capacity requirement. A solution used in long-distance transmission is costly. A capacity in an existing commercial short-distance solution soon cannot meet a requirement.
In a metropolitan-area/short-distance solution, a direct detection orthogonal frequency division multiplexing (direct detection orthogonal frequency domain multiplexing, DDOFDM for short) solution is promising. A principle of an orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing, OFDM for short) technology is: When a series of subcarriers whose frequency grid is Δf are modulated at a rate of Δf, the modulated subcarriers are mutually orthogonal. This process may be implemented by means of inverse fast Fourier transform (inverse fast fourier transform, IFFT for short) and fast Fourier transform (fast fourier transform, FFT for short), and a property of strong dispersion resistance may be achieved by adding a cyclic prefix (cyclic prefix, CP for short). However, subcarrier-subcarrier beating interference (subcarrier-subcarrier beating interference, SSBI for short) occurs during direct detection, severely affecting system performance. During transmission, an optical carrier and a modulated signal both are transmitted. A process of detection by an optoelectronic detector (PD) is essentially a process of detecting a beat between optical signals. At a transmit end, a detected optical signal mainly includes two parts: an optical carrier (Scarrier) and a modulated optical signal SOFDM. At a receive end, that is, at the optoelectronic detector, an output signal is proportional to an optical power P. P=(Scarrier+SOFDM) (S*carrier+S*OFDM)=Scarrier S*carrier+Scarrier S*OFDM+SOFDM S*OFDM, where Scarrier S*carrier is a direct current item, Scarrier S*OFDM is a beat item between an optical carrier and an optical signal that are expected to be obtained, and Scarrier S*OFDM is SSBI noise. The SSBI noise severely affects system performance, and therefore, how to reduce and eliminate the SSBI noise becomes a key technology for a DDOFDM system.
In the prior art, two methods for reducing subcarrier crosstalk are provided:
First: At a transmit end, signals are filled only on a band whose spectrum range is [W, 2W], where W is a bandwidth occupied by a modulated signal. In this case, at a receive end, a spectrum range for distribution of SSBI noise is [0, W], and the entire noise does not overlap the signal so that the signal can avoid being affected by the SSBI noise.
Second: During signal transmission, data is filled only on odd-numbered subcarriers. Herein, the subcarriers are marked as (2k+1)Δf (k=1, 2, and then SSBI is on subcarriers that are of frequency differences between the odd-numbered subcarriers and that are marked as 2mΔf (m=1, 2, 3 . . . ). It can be seen that a detected signal and SSBI noise are separately located on different subcarriers, that is, the signal is located on an odd-numbered subcarrier, and the SSBI is located on an even-numbered subcarrier, thereby avoiding impact from the SSBI noise.
In an implementation of the solutions in the prior art, it is found that the prior art has the following technical problem:
At both a transmit end and a receive end, a double signal bandwidth is required to eliminate SSBI noise, and bandwidth utilization is low.